It is generally accepted that brake system defects are responsible for a high number of automobile accidents. Results published by the Department of Transportation (Report No. DOT-HS-034-3-535-77-TAC; Mar. 31, 1977) indicate that a motor vehicle's braking system is the single highest cause of motor vehicle accidents.
Analysis by the Indiana University Institute for Research in Public Safety of the accidents reported for the five year period covered by the report (1972-77) indicate that the vehicle's braking system was "definitely" (95% confidence level) at fault in 2.9% of the accidents, "probably" (80% confidence level) at fault in 5.2% of the accidents, and "possibly" (20-80% confidence level) at fault in close to 20% of the accidents.
As evidence of state government concern, brake inspection and testing is given the highest priority in the various state motor vehicle inspection (MVI) programs. Due to the high volume of safety lane traffic, however, only a "full braking" dynamic performance test is used to determine MVI performance as opposed to visual inspection of the vehicle brake pads after removing the wheel ("wheel-pull inspection"), or a road test. Although wheel-pull or road test provide a much more quantitative result, they consume excessive inspection time and increase inspection costs.
The full brake test measures the vehicle's stopping power. Typically the vehicle's wheels are placed on a constant speed, torque motor driven pair of rollers. The brakes are applied and the magnitude of the torque drive required to rotate the rollers with the brakes applied is used as the indication of vehicle brake force. While this provides an excellent indication of stopping power it does not provide accurate measurement of the brake force imbalance between wheels on a common axle of the vehicle.
As known, brake imbalance causes a vehicle to swerve in hard stops, or to skid on slick road surfaces. The most significant cause of inaccurate measurement of brake imbalance in the prior art test procedures is the loss of brake shoe-to-drum, or brake pad-to-disc, contact as the wheel rotates. This is due to the out-of-roundness of the brake drum or to undulations in the disc surface. This unevenness characteristic is generally referred to as brake "ovality", which is defined as that effect which produces inconsistencies on a normal brake force curve. This masks the true imbalance magnitude.